Theoretical Revisit of the Direct Synthesis of H2O2 on Pd and Au@Pd Surfaces: A Comprehensive Mechanistic Study

The direct synthesis of H 2 O 2 from H 2 and O 2 on Pd(111) and Au@Pd(111) surfaces is studied with periodic density functional theory calculations. Ten possible reactions and processes involved in the H 2 O 2 synthesis steps are considered: For O 2, (1) O 2 * + H* → OOH*, (2) O 2 * → 2O*, and (3) O 2 * → O 2; for OOH, (4) OOH* + H* → H 2 O 2 *, (5) OOH* + H* → H 2 O* + O*, (6) OOH* + H* → 2OH*, (7) OOH* → O* + OH*, and (8) OOH* → OOH; for H 2 O 2, (9) H 2 O 2 * → 2OH* and (10) H 2 O 2 * → H 2 O 2, where the asterisks indicate these species to be surface species. All side reactions involve O–O bond dissociation. On the Pd(111) surface with H atoms coadsorbed, O 2 dissociation is suppressed; OOH dissociation is more favorable than all OOH hydrogenation reactions; three OOH hydrogenation reactions have comparable activation barriers; the barrier for H 2 O 2 dissociation is also comparable to that for H 2 O 2 desorption. However, on the H atoms coadsorbed Au@Pd(111) surface, the main reactions for H 2 O 2 production exceed all side reactions. The competition between the main reactions and the side reactions is actually the competition between the O–O bond and the O–M bond, where M is Pd in the case of the Pd(111) surface and Au in the case of the Au@Pd(111) surface. The O–Pd bond is usually stronger than the O–O bonds in the OOH intermediate and H 2 O 2; however, the O–Au bond is weaker than the O–O bonds. Consequently, the final product H 2 O 2 is easily produced and released from the Au@Pd(111) surface, and the side reactions involving O–O bond dissociation are suppressed. The role of the metal surface in the direct synthesis of H 2 O 2 from H 2 and O 2 is to provide H atoms as the feedstock for the hydrogenation of O 2 .